Onion Tart with Mustard and Fennel
Adapted from Gourmet
Serves about 8 as an appetizer

I like to slow cook my onions in a cast iron skillet. I find the evenness and the retention of the heat works really well. I would avoid a non-stick pan as it can take longer for them to caramelize. The round of parchment helps keep the onions really moist and takes them to a jammier texture but you can omit that step. Just make sure the heat is nice and low, stir them occasionally, and allow them at least an hour (or more) to achieve their dreamy potential.

Stir together yeast and warm water in a small bowl and let stand until foamy, about 5 minutes. (If mixture doesn’t foam, start over with new yeast.)

Put 1 ½ cups flour in a medium bowl, then make a well in center of flour and add yeast mixture to well. Stir together egg, 1 tablespoon oil, and 1 ½ teaspoons salt with a fork. Add egg mixture to yeast mixture and mix with a wooden spoon or your fingertips, gradually incorporating flour, until a soft dough forms. Transfer dough to a floured surface and knead, working in additional flour (up to ¼ cup) as necessary, until smooth and elastic, about 5 minutes. (DT: I used a lot of flour on my board to keep the dough from sticking.) Transfer dough to an oiled bowl and turn to coat. Cover with plastic wrap and let rise in a draft-free place at warm room temperature until doubled, 1 to 1 ½ hours.

While dough rises, heat remaining 1/3 cup oil in a 12-inch heavy skillet over medium-high heat until it shimmers, then sauté fennel seeds until a shade darker, about 30 seconds. Stir in onions, remaining teaspoon salt, and 1/2 teaspoon pepper, then reduce heat to medium-low and cover onions directly with a round of parchment paper. Cook, stirring occasionally, until onions are very tender and golden brown, 1 to 1¼ hours.

Preheat oven to 375°F with rack in middle.

Knead dough gently on a floured surface with floured hands to deflate. Transfer dough to a heavy baking sheet lined with parchment paper. Pat out dough into a roughly 15- by 12-inch rectangle, turning up or crimping edge, then brush mustard evenly over dough, leaving a ½-inch border around edge. Spread onions evenly over mustard, then sprinkle evenly with cheese.

Bake tart until crust is golden brown, 30 to 35 minutes. Cut into 2-inch squares or diamonds and serve warm or at room temperature.